In recent years, improvement in fuel consumption of automobiles has become an important problem from the viewpoint of global environment conservation. Therefore, there has been an active movement for thinning car body materials by increasing the strength thereof, thereby lightening the weights of car bodies. However, an increase in strength of steel sheets causes a decrease in elongation, i.e., a decrease in formability, and thus development of materials having both high strength and high formability is demanded.
Further, in consideration of recent increases in demands for improvement of corrosion resistance of automobiles, high-strength galvanized steel sheets have been increasingly developed.
For these demands, various multi-phase-type high-strength galvanized steel sheets, such as ferrite-martensite two-phase steel (DP steel) and TRIP steel using the transformation-induced plasticity of retained austenite, have been developed so far.
For example, Patent Literature 1 proposes a galvannealed steel sheet with excellent formability which contains a large amount of Si added to secure retained austenite and achieve high ductility.
However, the DP steel and the TRIP steel have excellent elongation properties but have the problem of poor stretch flangeability. The stretch flangeability is an index which indicates formability (stretch flangeability) in forming a flange by expanding a formed hole and is an important characteristic, together with elongation, required for high-strength steel sheets.
As a method for manufacturing a galvanized steel sheet having excellent stretch flangeability, Patent Literature 2 discloses a technique for improving stretch flangeability by reheating martensite to produce tempered martensite, the martensite being produced by annealing and soaking and then strongly cooling to a Ms point during the time to a galvanization bath. Although the stretch flangeability is improved by converting martensite to tempered martensite, low EL becomes a problem.
Further, as a performance of press-formed parts, the parts include portions required to have fatigue resistance, and thus it is necessary to improve the fatigue resistance of materials.
In this way, high-strength galvanized steel sheets are required to have excellent elongation, stretch flangeability, and fatigue resistance. However, conventional galvanized steel sheets do not have high levels of all these characteristics.